M=3.5 earthquake strikes near creeping San Andreas and Calaveras Faults

The San Andreas Fault cuts through DeRose Winery in Hollister, CA. The resulting creep (steady slip) has progressively offset this culvert over the past 30 years. (Photo from geologycafe.com)

At 9:35 p.m. local time last night (January 30), a small M=3.5 earthquake shook the city of Hollister, CA, and was felt as far away as Santa Cruz and Monterey. According to the USGS ShakeMap, only light shaking was felt, and around 300 people recorded feeling the quake on the USGS website.

This earthquake occurred between the creeping sections of the San Andreas and Calaveras faults. The fact that they are creeping means that there is very slow continuous motion along the faults. Areas such as this do not tend to have large earthquakes as stress does not build up as much as it does on locked sections.

Even though this earthquake occurred close to the San Andreas and Calaveras faults, based its focal mechanism, it appears to have occurred along a secondary fault. We would expect an earthquake on either the San Andreas or Calaveras faults to have almost pure strike-slip motion. While this quake was primarily strike-slip, it did also have a large component of extensional motion. Nonetheless, because of its location, it merits a closer look.

Because this earthquake occurred in the creeping section of two major faults, we thought we’d check to see if this could be part of a larger creeping event. However, the closest GPS station to the epicenter did not pick up any movement. Therefore, we believe this was an isolated event not associated with larger movement. Having said that, we will monitor the area and if there are more earthquakes in the section, we will be sure to update the situation.

This Temblor map shows that the epicenter of the M=3.5 earthquake on January 30 has a Temblor Hazard Rank of 99. This means the area is extremely susceptible to shaking. This is due to the convergence of the San Andreas and Calaveras faults.

The exact location of this earthquake has a Temblor Hazard Rank of 99. This means it is one of the most earthquake-prone areas in the country, and has a high likelihood of experiencing strong shaking. So, a small quake here is not a surprising event, and residents of the area are accustomed to shaking.